1. Industrial Application of the Present Invention
The present invention relates to an information recording medium formed in a card shape (hereinafter simply called an optical card) wherein information is recorded on and reproduced from a prescribed track in accordance with optical scanning, and, in particular, to an optical card wherein an access operation to a desired track can be performed reliably when information is to be recorded or reproduced.
2. Prior Art
A conventional optical card records information in a prescribed rectangular region of longitudinal dimension L and vertical dimension W on one surface of a card of the same external dimensions as those of a credit card, as shown by way of example in FIG. 4.
This region is provided with an information recording section 1 in the center, with preformatting sections 2 and 3 to left and right and mirror sections 4 and 5 above and below the information recording section 1.
The information recording section 1 and the preformatting sections 2 and 3 are, as shown in FIG. 5, formed of guide tracks 6 that are, for example, 2.5-.mu.m wide at a pitch of 12 .mu.m. These guide tracks 6 have an optical reflectivity that is less than that of other sections of the card. Information tracks 9 of a width of 9.5 .mu.m are provided sandwiched between the guide tracks 6.
To record information onto an information track 9, the information is recorded by irradiating a laser beam onto an information track region 7 provided along the center of the information track 9 to form information pits 8 therein, while the optical card is being moved parallel to the guide tracks 6. To reproduce recorded information, a laser beam is irradiated onto the information track 9 which comprises a string of information pits 8, and the strength of the light reflected back therefrom is reproduced as information. The information pits 8 have an optical reflectivity less than that of other parts of the optical card, and thus the strength of the light reflected back therefrom is weaker, and the information can be reproduced as differences in this strength.
The preformatting sections 2 and 3 are provided at either end of the information recording section 1 in the scanning direction and contain previously recorded address information. The mirror sections 4 and 5 are provided on either side of the information recording section 1, parallel to the scanning direction. These mirror sections 4 and 5 are provided in order to protect the guide tracks 6 of the information recording section 1.
When information is recorded on the optical card configured in this manner, or reproduced therefrom, a laser beam must be irradiated onto the information tracks 9, so tracking is necessary to ensure that it does not deviate from the information tracks 9.
Tracking is done by a method such as a push-pull method or a three-beam method. With the push-pull method, as shown in FIG. 6(a), a main beam 10 of a pair of laser beams is irradiated onto an information track 9 and a secondary beam 11 is irradiated onto the center of a guide track 6, the distribution of strength across the width of the resultant reflected beam is detected, and the system is controlled in such a manner that the strength distribution is kept symmetrical in the direction perpendicular to the guide track 6 to ensure that the secondary beam 11 does not deviate from the center of the guide track 6. This also controls the main beam 10 in such a manner that it does not deviate from the information track 9.
In the three-beam method illustrated in FIG. 6(b), a main beam 10 is irradiated onto the information track 9 and half each of two secondary beams 11a and 11b is irradiated onto adjoining guide tracks 6a and 6b. Control is such that the difference in strength of the light reflected back by the secondary beams is made to be zero, and thus the linked main beam 10 is controlled in such a manner that it does not deviate from the information track 9.
These conventional methods of tracking use the guide tracks 6 as reference. Therefore, if there are no guide tracks 6, the positions at which the laser beams (the main beam 10 and the secondary beam(s) 11) irradiate cannot be controlled, and thus the laser beams will irradiate an undesirable area.